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CINCINNATI (AP) — Awakened by his first call to the majors on Friday morning, top Chicago Cubs prospect Starlin Castro shook off sleep and disbelief to become an immediate star.

The 20-year-old became the youngest shortstop to make his Cubs debut when he stepped on the field about 12 hours later, and that was just the start of an historic night.

Castro hit a three-run homer off Cincinnati's Homer Bailey in his first at-bat, making him the sixth player in Cubs history to connect his first time up. The last to do it was Jim Bullinger in 1992.

Three innings later, Castro hit a bases-loaded triple to the gap in right-center, sliding headfirst into the record books again. The six RBIs were the most for any player in a modern-day debut. Four others had five RBIs, most recently Ben Grieve in 1997.

Already, a Cubs sensation.

He made the jump from Double-A Tennessee, where he was asleep after a night game when the phone rang at 7 a.m. on Friday. The Cubs' top-rated prospect didn't expect to be in the majors until much later in the season.

A few hours later, he was in Cincinnati and in the starting lineup for a game against the Reds, batting eighth as the Cubs let him get acclimated.

"I'm happy. Whoo!" he said, sitting in his locker with a No. 13 jersey hanging from the side.

Castro became the youngest Cub to make his big league debut since Oscar Gamble did it at the age of 19 in 1969. He also was the youngest shortstop to make his debut with the Cubs.

Chicago optioned infielder Chad Tracy to Triple-A Iowa to make room on the roster. Tracy opened the season with the Cubs and batted .273 in 19 games.

The Cubs made the move after they got swept in a three-game series in Pittsburgh, dropping them to 13-16 — good for fourth place in the NL Central. It's the second major move the Cubs have made to try to get their season turned around.

Manager Lou Piniella shifted ace Carlos Zambrano to the bullpen on April 21, trying to fix their biggest problem at that point. A shaky bullpen had threatened to scuttle their season less than a month into it.

The bullpen has improved with Zambrano. Now, the Cubs have to do something to improve the everyday lineup for now and later.

"Look, can't look at this thing day to day," Piniella said. "It gets too tiring. It gets too cumbersome. You've got to look over a period of time. You've got to look at things with a little longevity to them. That's what we're doing with this move."

Castro went to spring training with the Cubs, wearing No. 67. He expected to spend most of the season in the minors, but sped up the process by batting .376 with eight doubles, five triples and a homer in 26 games at Tennessee.

When he got awakened by the call to the majors, he was surprised.

"I said, 'Are you serious?'" Castro said.

He then called his family and friends in the Dominican Republic. The native of Monte Cristy signed with the Cubs as a free agent on Oct. 25, 2006. He batted .310 with 48 doubles, 18 triples, 9 homers and 122 RBIs in four seasons in the minors.

Piniella was impressed by how Castro handled himself during spring training, when the shortstop turned 20.

"This spring if you didn't mention that he was 19 years old, you couldn't tell it by the way he handled himself," Piniella said.

Piniella had a talk on Friday with Ryan Theriot, who moves to second base to open a spot for Castro. Theriot became the Cubs' everyday shortstop in 2007. He started 147 games there last season, the most by a Cub since Shawon Dunston started 150 games at shortstop in 1988.

Theriot, the Cubs' leading hitter, has played in 79 games at second base during his career.

"He's played there before," Piniella said. "He had a little workout there this afternoon and we'll throw him into the fire."